Am_Fear_Liath_Mor is referring to Chernobyl's decision to test Plan C (diesel generators) for powering the reactor cooling pumps. There was a 30-second startup delay in the diesels and incredibly, no one had thought to see if that was a problem during the design or provisioning of the plant. So they decided to test it at a scheduled shutdown. Failure of grid power (Plan A) and the plant's normal generation gear (Plan B) is what Plan C is for.

The test failed but that didn't cause the disaster - they were already at low power. The power excursion happened when they tried to shut down the reactor. They were doomed an hour or more earlier when the control rods were nearly fully removed and no one wondered if getting minimal reactor power with the controls wide open was a warning sign...

It likely would have gone boom! if they had just shut down the reactor instead of starting the diesel generator test but not if they had been at normal operating power (maybe damaged the core, but no boom!) or if a bunch of other things had been done differently.

The bottom line is that even Soviet reactors of the Chernobyl-era could cope with loss of grid power and loss of their own normal power generation. What they couldn't cope with is being run below minimum power for hours and then suddenly shut down.